//
// Created by 谢哲 on 2021/4/18.
//

#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;

int main() {
    double a, b, c;
    cin >> a >> b >> c;

    double p = 0.5 * (a + b + c);

    double s = sqrt(p*(p-a)*(p-b)*(p-c));
    int ans = int(s * 10.0 + 0.5);

    cout << fixed << setprecision(1) << double(ans) / 10.0;

    return 0;
}